tt: add ignoring of the SIGURG signal sending by go runtime

This patch fixes the problem, which was occurring when the watchdog
process received a signal SIGURG from go runtime[1][2] and passed it
to the forked process before the exec call. Fixed by adding a call of
the Ignore function. Receiving this signal is unexpected, cause tt
doesn't work with sockets at all.

[1] - https://go.googlesource.com/proposal/+/master/design/24543-non-cooperative-preemption.md
[2] - golang/go#37942

Closes #325

Author: vr009

cli/running/watchdog.go

@@ -113,6 +113,13 @@ func (wd *Watchdog) startSignalHandling() {
 	sigChan := make(chan os.Signal, 1)
 	signal.Notify(sigChan)
 
+	// This call is needed to ignore SIGURG signals which are part of
+	// preemptive multitasking implementation in go. See:
+	// https://go.googlesource.com/proposal/+/master/design/24543-non-cooperative-preemption.md.
+	// Also, there is no way to detect if a signal is related to the runtime or not:
+	// https://github.com/golang/go/issues/37942.
+	signal.Ignore(syscall.SIGURG)
+
 	// Set barrier to synchronize with the main loop when the Instance stops.
 	wd.doneBarrier.Add(1)